In general, a tuner receives a radio frequency (RF) signal through an antenna, converts the RF signal into an intermediate frequency (IF) signal, detects the IF signal, separates the detected IF signal into a video signal and a sound signal, and outputs each signal. There is an advantage of improving frequency selectivity, reducing sensitivity for peripheral circuits and enhancing stability by additionally performing the IF conversion process during an operation of the tuner.
A structure of a general tuner will be briefly described below. The general tuner includes an antenna, a mixer, and a local oscillator. A TV broadcasting system divides its wide RX frequency band (50-900 MHz) into a VHF band and a UHF band and uses two corresponding band blocks (that is, VHF/UHF band blocks) because its tuner uses a variable capacitance diode having a limited capacitance ratio. The VHF/UHF band blocks each include an input unit, an amplifier unit, and a tuning unit.
An operation of the tuner will be briefly described below. In the tuner, the antenna receives RF signals and transmits the RF signals to the band block. In the band block, the input unit selects and outputs only a desired band RF signal among the RF signals from the antenna, the amplifier unit amplifies the desired band RF signal from the input unit, and the tuning unit tunes and outputs the RF signal received from the amplifier unit.
The mixer mixes the RF signal received from each band block with an oscillation frequency signal to output an IF signal. The oscillation frequency is received from the local oscillator.
Various types can be applied to the TV broadcasting system. The present invention mainly relates to an NTSC TV broadcasting system but is not limited to the NTSC TV broadcasting system.
In the general tuner, a TV video signal is influenced by an FM radio signal when the US channel 6 is selected. In more detail, the US channel 6 has a video frequency of 86.83 MHz and a sound frequency of 87.75 MHz. An FM radio frequency signal of 88.1-88.5 MHz most affects the US channel 6. The FM radio frequency signal of 88.1-88.5 MHz adds a noise signal to a noise signal of the US channel 6 because it is adjacent to a frequency band of the US channel 6.
In the general tuner, a trap filter is connected to a tuning unit of the VHF band block so as to eliminate the FM radio frequency signal of 88.1-88.5 MHz and. When the US channel 6 is selected, the trap filter operates according to a control signal of a microcomputer or an external switching signal so as for the tuning unit to directly eliminate the FM radio frequency signal of 88.1-88.5 MHz. Accordingly, the trap filter directly eliminates the FM radio frequency signal of a high frequency band, and the mixer converts the resulting signal into an IF signal.
However, when the FM radio frequency signal of a high frequency band is eliminated, the video and sound frequency signals of the US channel 6 adjacent to the FM radio frequency signal is undesirably weakened. In more detail, when the US channel 6 is selected, the trap filter eliminates the FM radio frequency signal of 88.1-88.5 MHz. At this time, the video frequency signal of 86.83 MHz and the sound frequency signal of 87.75 MHz in the US channel 6 adjacent to a trap frequency are also eliminated, and thus the video and sound quality is deteriorated.